The present invention relates to an antenna apparatus and, in particular, a small antenna array that generates an endfire pattern in a desired direction while simultaneously forming a null in the opposite direction. Employing the particular antenna apparatus in a larger antenna array of many elements provides a wide field of view by increasing the scanning ability of the array element to near grazing angles.
Antenna array systems for transmitting and/or receiving data or other information have been devised in a variety of configurations. Phased array antenna systems require many costly components that contribute to a design complexity that may not be acceptable or appropriate for certain situations. The most general implementation of a phased array produces an array design capable of focusing the energy from all antenna elements to any desired point in space. Phased array antennas have their elements arranged in rectangular or triangular grid lattices and are capable of focusing the antenna array pattern from broadside to the array to angles nearing 50 degrees off of broadside without difficulty. Scanning the array to angles exceeding 50 degrees becomes increasingly more difficult. In some applications, however, it may be desirable to operate an array in an endfire mode, which directs the radiation along the axis of the array at a scan angle of 0xc2x0, corresponding to 90xc2x0 from broadside.
Endfire operation is the most difficult mode in which to use a phased array. Upon attempting to use a phased array to scan in the endfire direction, several problems arise which severely limit the array""s ability to scan to angles approaching endfire. Traditional designs used for antenna arrays which are required to scan in the endfire direction call for very specialized antenna elements with limited fields-of-view (FOV). If an application requires an antenna array which is able to scan beyond the maximum scan angle of the antenna element, multiple arrays must be used. For example, if an application required 0xc2x0-90xc2x0 of scan angle, three arrays might be needed, one for scan angles of 0xc2x0-15xc2x0, another for scan angles of 15xc2x0-30xc2x0, and a third for scan angles beyond 30xc2x0. While the use of multiple arrays can increase the scan angle of the antenna system, it can increase the cost and complexity of the antenna system.
In addition to the scan angle limitations, traditional endfire antennas have other physical problems. For example, grating lobes will be generated if the inter-element spacing exceeds xcex0 /2 and the array is used to scan to angles exceeding a nominal value. This includes scanning the array to angles approaching endfire. A grating lobe is a lobe other than the main lobe produced by an antenna array when the inter-element spacing is sufficiently large to permit the in-phase addition of radiated fields in more than one direction. Grating lobes are undesirable because the antenna is less efficient due to the energy that is being directed in the direction of the grating lobe instead of in the desired direction of the main beam of the antenna pattern. Additionally, grating lobes result in possible target ambiguities and false targets which arc difficult for a radar to resolve. In order to reduce grating lobes produced by the application of an antenna for endfire applications, elements in an array are typically arranged such that the distance between the elements is less than one-half wavelength of the center operating frequency of the array (i.e. xcex0.) However, this element spacing constraint can increase the difficulty and cost to manufacture the array and increase the mutual coupling between elements causing increased mismatch with scan angle. Phased array antennas typically have certain components which are required for each element within the array. This hardware includes transmit and receive modules (T/R modules), phase shifters, low noise amplifiers, high power amplifiers, and limiters. If the elements are spaced relatively closely, as discussed above, this results in an antenna which is difficult and expensive to build, expensive to maintain, and may have reliability issues.
In addition to the cost and complexity of phased array antennas designed for use in endfire applications, the antenna systems ability to transmit and receive can also be degraded. As described above, scan limitations are common for phased array antennas scanning in an endfire mode. Scan limitations result from mutual coupling between elements in an array. Mutual coupling is the mechanism by which fields present at one element due to a forced excitation produce significant fields in other elements. Due to mutual coupling, a fraction of the energy incident on each element in the array will be scattered off the elements in all directions, allowing the elements themselves to behave as secondary radiators. Mutual coupling results in an active impedance which is a function of scan angle. If the active impedance of the elements in an array is not controlled by some means, large reflection coefficients will result and the individual elements will reflect power that is incident on them from the transmitter. In other words, the antenna will not transmit the power input into the phased array antenna. The reflection coefficient is the ratio of reflected to forward voltage at a specified reference plane. In traditional array antennas as the scan angle approaches endfire, the active impedance causes the reflection coefficient to increase towards a value of one. As the reflection coefficient approaches a value of one, only a very small percentage of the power input into the array is transmitted, while the remaining power is reflected back to the transmitter. Additionally, the reflected power creates heating within the antenna, which must be dissipated.
When scanning an array antenna, it is desired to have the magnitude of the reflection coefficient as small as possible. In many applications, an acceptable magnitude of the reflection coefficient is approximately 0.33, which results in a voltage standing wave ratio (VSWR) of 2:1. For an application with a FOV extending from 0xc2x0 to 90xc2x0 (where 90xc2x0 corresponds to the plane of the array), the VSWR must be below 2:1 for satisfactory performance. Another way to consider the detrimental effects of excessive reflection coefficient is to consider the effective transmission loss due to reflection coefficient. An effective transmission loss can be computed for any value of reflection coefficient. Transmission loss is a measure of output power compared to input power and can be measured in dB by taking 10 log10(x) where x is the ratio of output power over input power.
The net result of the uncontrolled active impedance of N elements in an array antenna is to produce an excessive VSWR or excessive transmission loss at specific angles over the FOV the antenna will be used to scan through, as well as a trend of severely degraded performance over angles nearing endfire. The occurrence of high VSWR at specific angles is referred to as scan blindness. Traditionally the effects of scan blindness have been dealt with by mitigating the effects of high VSWR by designing the array with a lattice structure favoring performance over some regions while compromising performance over other regions. Although design measures can be taken to mitigate scan blindness effects and the effects of severely degraded performance as an array is scanned near endfire, technology has not been available to altogether eliminate or reduce these effects to a satisfactory level.
In addition to active impedance performance of an array another very important characteristic of the array is the reduced radar cross section (RCS). Reduced RCS is directly attributed to the reduction in impedance mismatch or reflection coefficient as an array antenna is scanned throughout its FOV.
In accordance with the present invention, an apparatus is disclosed for providing an array antenna capable of scanning zero through ninety degrees. The apparatus includes one or more rows of antenna elements. Each row of antenna elements is arranged such that the antenna elements are in pairs, with a first pair having a first and a second antenna element and a second pair having a third and fourth antenna element. It is important that antenna element pairs be isolated from each other (no, or substantially no, interference or other effect by one pair of antenna elements on another pair of antenna elements during energization or other use thereof) not that one antenna element of a pair be isolated from the other antenna element of the pair. Each antenna element of the same pair is also excited with a single feed point. An antenna element may be comprised of one or more parts, components or sub-clement, all of which are required to achieve proper functioning of the antenna elements. For example, one dipole which has more than one integral part is one antenna element, not two antenna elements. In one embodiment, the antenna elements have a longitudinal center axis, and the antenna elements are laterally spaced from each other in a direction substantially perpendicular to the longitudinal center axes. A first lateral distance is defined between the longitudinal center axes of the first and second antenna elements and a second lateral distance is defined between the longitudinal center axes of the second and third antenna elements, with the second lateral distance being greater than the first lateral distance. The apparatus may have transmit and/or receive (T/R) module circuitry electrically connected to each pair of antenna elements that transmits and receives signals. In this case the apparatus would have a control system that controls activation and deactivation of the transmit and/or receive module circuitry, with the control system controlling generation of a scanning beam output by the antenna apparatus. The apparatus may also be used in a simpler array architecture not requiring transmit and/or receive circuitry at each pair, but instead with multiple element pairs combined with a passive beamforming network.
A first halfway point is defined between the longitudinal center axes of the first and second elements and a second halfway point is defined between the longitudinal center axes of the third and fourth antenna elements. A pair separation distance is defined between the first and second halfway points. Lambda (xcex0) is defined as the wavelength of the center frequency of signals transmitted by the array antenna.
In one embodiment, the pair separation distance is greater than xcex0/two, and the lateral distance between the first and second elements within a pair equals about xcex0/four. The first and second antenna elements within a pair are arranged to be 90xc2x0 out of phase with one another and have a phase quadrature relationship so that the pair of antenna elements generates a scanning beam in a first direction and a controlled pattern null in a desired direction. Each pair of antenna elements may be a different type of element radiator such as a pair of slot antenna elements, a pair of micro-strip patch antenna elements, a pair of monopole antenna elements or a pair of dipole antenna elements.
In one embodiment, the first and second antenna elements are slot antenna elements and are formed in a body member, and are coupled together using a coupling structure. The T/R module circuitry includes a T/R module operably connected to the coupling structure. The coupling structure has a midpoint, and the first T/R module is connected to the coupling structure offset from the midpoint, resulting in an offset signal between the two antenna elements. The difference in length of the coupling structure between elements is xcex0/four, resulting in the signal offset at the antenna elements being 90xc2x0.
Based on the foregoing, several benefits of the present invention are readily seen. The antenna apparatus can generate a scanning beam which is movable between at least 0xc2x0-90xc2x0, where 0xc2x0 is defined along a plane parallel to the first row of antenna elements and 90xc2x0 is defined along a plane perpendicular to the first row of antenna elements. While scanning through 0xc2x0-90xc2x0, the magnitude of the reflection coefficient produced by the scanning beam is desirably low. Additionally, the transmission loss of the scanning beam is reduced.
Additional advantages of the present invention will become readily apparent from the following discussion, particularly when taken together with the accompanying drawings.